A Petroleum Geologist s. Guide to Seismic Reflection. William Ashcroft

Transcription

1 A Petroleum Geologist s Guide to Seismic Reflection William Ashcroft

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3 A Petroleum Geologist s Guide to Seismic Reflection

4 Companion CD-ROM The computer-aided tutorials in the book are written for the PC and should run in any version of Microsoft Windows from Windows 95 to Windows 7. Most of the programs read and write files to disk, so the simplest way to use them is to copy all files off the CD to a folder on disk. To run a program simply list the files in that folder in Windows Explorer and double click on the appropriate.exe file or right click on the.exe file to create a shortcut. The workings of each program are explained under the menu heading Help/Quickstart or Help/Manual. Double click on STARTUP.EXE for further information. One tutorial requires the use of a spreadsheet in Windows Excel.

5 A Petroleum Geologist s Guide to Seismic Reflection William Ashcroft

6 This edition first published 2011, Ó 2011 by William Ashcroft Blackwell Publishing was acquired by John Wiley & Sons in February Blackwell s publishing program has been merged with Wiley s global Scientific, Technical and Medical business to form Wiley-Blackwell. Registered office: John Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester, West Sussex, PO19 8SQ, UK Editorial offices: 9600 Garsington Road, Oxford, OX4 2DQ, UK The Atrium, Southern Gate, Chichester, West Sussex, PO19 8SQ, UK 111 River Street, Hoboken, NJ , USA For details of our global editorial offices, for customer services and for information about how to apply for permission to reuse the copyright material in this book please see our website at The right of the author to be identified as the author of this work has been asserted in accordance with the Copyright, Designs and Patents Act All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by the UK Copyright, Designs and Patents Act 1988, without the prior permission of the publisher. Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic books. Designations used by companies to distinguish their products are often claimed as trademarks. All brand names and product names used in this book are trade names, service marks, trademarks or registered trademarks of their respective owners. The publisher is not associated with any product or vendor mentioned in this book. This publication is designed to provide accurate and authoritative information in regard to the subject matter covered. It is sold on the understanding that the publisher is not engaged in rendering professional services. If professional advice or other expert assistance is required, the services of a competent professional should be sought. Library of Congress Cataloguing-in-Publication Data Ashcroft, W. A. (William A.) A petroleum geologist s guide to seismic reflection / William Ashcroft. p. cm. Includes index. ISBN (cloth) ISBN (pbk.) 1. Seismic reflection method. 2. Petroleum Prospecting. I. Title. II. Title: Guide to seismic reflection. TN A dc A catalogue record for this book is available from the British Library. This book is published in the following electronic formats: epdf [ISBN: ]; epub [ISBN: ] Set in 9/11.5pt Minion by Thomson Digital, Noida, India

7 Contents Preface Acknowledgements xi xiii Part I Basic topics and 2D interpretation 1 Introduction and overview Exploration geophysics in petroleum exploration The principle of seismic reflection surveying Overview of the seismic reflection industry A brief history of seismic surveying Societies, books and journals 5 2 Geophysical signal description Overview Cosine waves Signals and spectra Periodic waveforms: Fourier series Seismic wavelets Wavelet characteristics: time and frequency domains Digitization of signals Fourier description of space-dependent quantities 13 Tutorials: 13 Tutorial 2.1 Consolidating ideas of frequency and phase shift 13 Tutorial 2.2 Fourier summation of a periodic waveform 14 Tutorial 2.3 The transition from periodic waveform to wavelet: program FOURSYN 15 Tutorial 2.4 Amplitude and phase-shift changes in the wavelet: program FOURSYN 15 3 Data acquisition General points Seismic sources and receivers Static corrections Recording and presentation of data Common mid-point (CMP) shooting The attack on noise D surveys 22 4 Seismic wave propagation Introduction P-wave Controls on P-wave velocity 24

8 vi Contents 4.4 P-wave waveforms Shear waves and surface waves P-wave attenuation Spherical spreading Frequency-dependent attenuation P-wave transmission paths Wavefronts and raypaths for a single interface Reflection coefficient and wavelet polarity Ray-tracing in seismic modelling 29 Tutorials: 30 Tutorial 4.1 P- and S-wave particle motion on screen: program PSWAVE 30 Tutorial 4.2 Basic measurements of time, velocity and depth 30 Tutorial 4.3 Drawing the reflection wavefront from Huygens s Principle 30 Tutorial 4.4 Calculating typical reflection coefficients from well data 31 5 The process of reflection Introduction Fresnel zones Fresnel zones and the seismic reflection Faults and diffractions Hyperbolae on stacked time sections The reflection as a summation of hyperbolae Resolution of the seismic reflection method Multiple reflections: common modes Multiples: the scale of the problem 38 Tutorials: 39 Tutorial 5.1 Seismic expression of a point reflector 39 Tutorial 5.2 Water-layer multiples spoil a deep reservoir interval: program CMPGATHER 39 6 Velocity analysis, CMP stacking and post-stack migration General points Definitions of seismic velocity: well data Velocities from seismic data: V rms Velocities from seismic data: V stack Velocity analysis Errors in seismic-derived velocities Multiple suppression by CMP stacking Stacking the whole section: a make-or-break process Some stacking refinements NMO stretch Weighted stacking Migration: the fundamental idea Map migration Full-waveform migration Migration by Kirchhoff diffraction stack Migration by wave equation Migration example: 2D section 48 Tutorials: 49 Tutorial 6.1 Velocities from well data 49 Tutorial 6.2 NMO correction, CMP stacking and velocity analysis: program NMOSTAK 50 Tutorial 6.3 Picking stacking velocities from a velocity spectrum: programs VELSPEC and SEGY2D 51 Tutorial 6.4 Suppression of multiples by CMP stacking: program NMOSTAK 51 Tutorial 6.5 How multiples appear on a velocity spectrum: program VELSPEC 51 Tutorial 6.6 Migration by ray-tracing 52

9 Contents vii 7 Interpretation of two-dimensional (2D) surveys for structure Introduction Linking well geology to the seismic section Sonic log or continuous velocity log (CVL) Time-depth plot Making the link: synthetic seismogram Choosing reflections to pick Picking reflections Sideswipe A sideswipe example: fault diffractions Preparing structure maps in TWT Time to depth conversion Velocity as an analytical function of depth Time-depth conversion strategies Examples of time-depth conversion Southern North Sea: Rotliegend sandstone target Central North Sea: Paleocene sands target West Sole field, southern North Sea 60 Tutorials: 60 Tutorial 7.1 Constructing a synthetic seismogram from well-log data: program SYNTH 60 Tutorial 7.2 Matching a synthetic seismogram to seismic data: program IMAGES 62 Tutorial 7.3 Picking reflections along a 2D section from the Moray Firth, northern North Sea basin 62 Tutorial 7.4 Time to depth conversion, West Sole Field, southern North Sea Basin 70 Part II Seismic input to reservoir characterization 8 Better images of the subsurface Introduction Reflection point dispersal, conflicting dips and DMO Prestack time migration (PSTM) Common-offset sections and the Cheops pyramid PSTM and image gathers The limitations of PSTM: lateral variations in velocity Prestack depth migration (PSDM) Velocity-depth model based on layers Velocity-depth model based on tomography Anisotropy: the ultimate refinement in velocity Velocity-depth ambiguity Future migration technique: Kirchhoff or wave extrapolation? D migration D seismic interpretation Growth and impact of 3D seismic surveys 94 Tutorials: 95 Tutorial 8.1 Reflection point dispersal 95 Tutorial 8.2 Lateral mis-location from time migration 96 Tutorial 8.3 3D data: vertical section and time-slice 97 9 Modifying the seismic waveform Introduction Testing an electronic filter: the impulse response Digital filters: convolution Cross-correlation and auto-correlation Frequency filtering by convolution 102

10 viii Contents 9.6 The seismogram as a convolution Deconvolution Designing deconvolution operators Predictive deconvolution Wavelet processing Frequency-domain processing Data processing and the fragility of bandwidth 106 Tutorials: 107 Tutorial 9.1 Digital filtering by hand 107 Tutorial 9.2 The power of the Vibroseis technique: program SIGPROC 107 Tutorial 9.3 Testing the seismic response of a geological model: program SYNTH Refining reservoir architecture from seismic data Introduction: the reservoir model Refining reservoir environment: seismic stratigraphy and facies analysis Sequences and system tracts Picking seismic sequence boundaries Seismic facies analysis Sedimentary units as 3D volumes Multi-attribute facies analysis Analysis of seismic facies by trace shape Seismic facies in carbonates Refining reservoir structure: vertical seismic profiling (VSP) VSP processing and applications Walkaway VSP Refining reservoir structure: seismic attributes Horizon displays of dip magnitude and azimuth Volumetric dip magnitude and azimuth Coherence Automatic fault extraction: ant-tracking Curvature Applications of curvature Structure-oriented filtering Seismic forward modelling One-dimensional modelling: the synthetic seismogram Mis-match between synthetic seismogram and section Forward modelling in two and three dimensions 124 Tutorials: 126 Tutorial 10.1 Section limits in walkaway VSP 126 Tutorial 10.2 Forward modelling of fault shadow: program SYNTHSEC Seismic input to mapping reservoir properties Introduction Reflection amplitude Acoustic impedance (AI) inversion AI inversion by recursion and trace integration The good and the bad of AI inversion Sparse-spike, model-based and coloured inversion Amplitude variation with offset (AVO) AVO and poisson s ratio AVO methodology Angle stacks AVO intercept and gradient Intercept-gradient cross-plots Fluid factor AVO inversion to rock properties lr and mr AVO inversion to P- and S-wave impedance 138

11 Contents ix 11.9 Elastic impedance: AVO made easy? Best fluid indicator? Instantaneous seismic attributes Usage of seismic attributes Predicting log properties from seismic attributes C and 4D surveys 143 Tutorials: 144 Tutorial 11.1 AVO for typical lithological interfaces 144 Tutorial answers 146 References 147 Index 153 Files for computer-aided tutorials in the text are provided in the companion CD at the front of the book.

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13 Preface This book is written for anyone who wants to get quickly up to speed on some aspect of reflection seismology as it affects the seismic interpreter. It is a development of course notes on seismic reflection interpretation which have been given to students on the MSc course in Petroleum Geology at Aberdeen University over many years, and thus it takes the form of a course manual rather than a systematic textbook. It can be used as a self-contained course for individual study or as the basis of a class programme. The notes were originally provided to make the subject more accessible to geology students, but this volume should also prove useful to others, such as petroleum engineers, who have to work in an integrated exploration or development team side by side with geophysicists and geologists. Much petroleum exploration and production is now driven by the seismic reflection survey technique, so that all team members need to know quite a lot about it. Throughout this book, the way in which the subject matter is treated depends on its importance to the seismic interpreter. For example, in discussing data acquisition in the field, only the barest descriptions of seismic source and receiver hardware are given; the focus is on the geometry of survey layout and the maintenance of signal quality. Geophysics uses the language of mathematics, which is like any other language if you don t use it every day you soon forget it, so many people find the mathematics in geophysics a barrier to learning. The first rule for reading a maths-based topic might be summarized as don t panic! The second rule is to keep re-reading the bits you don t understand, with pencil and paper handy. The third rule is to read only twice over at any one time, then move on to rest your conscious brain and give its unconscious part time to work around it. That often works best while doing a practical tutorial of some sort maybe something as simple as plotting a graph. Let your fingers help the learning! The book is written with an eye to those points that students of the subject tend to find difficult, and it tries to provide insight through tutorial material of a practical nature. The tutorials aim to reinforce and deepen understanding of key topics and to provide the reader with a measure of feedback on progress. Some tutorials may only involve drawing simple diagrams, but many are computeraided tutorials with graphics output to give insight into key steps in seismic data processing, or into the seismic response of some common geological scenarios. Other tutorials involve interpretation of seismic sections and associated well data. The reader is urged to complete the practical tutorials at the time they are encountered in the text. The main interpretation tutorials at the end of Chapter 7 can be done over a longer schedule. There are two separate applications of the seismic reflection technique in the petroleum industry. The first is to determine subsurface geological structure as exactly as possible, by calculating the depth to key geological horizons and so delineate closed, possibly hydrocarbon-bearing structures and calculate their volume. The second application is to use seismic characteristics such as signal amplitude or frequency to determine subsurface properties such as porosity, or the presence of hydrocarbons in a reservoir rock, and to track variations in such properties away from well control. Part I of this book covers fundamental topics such as data acquisition and the description of the seismic wavelet, together with structural interpretation from two-dimensional seismic sections. Part II deals with three-dimensional surveys and the seismic input to reservoir studies. Dr. William Ashcroft, Aboyne, Royal Deeside, Scotland

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15 Acknowledgements I am grateful for helpful comments on drafts of this work from Alan McGregor, Peter McAllister-Hall, Henry Allen, Stephen Spencer and Adrian Burrows. Professor Jonathan Redfern, University of Manchester, helped the author to maintain focus on seismic facies. Seismic data for interpretation purposes was generously provided by WesternGeco and BP. The open policy of the Society of Exploration Geophysicists towards the use of illustrative material from their publications has been a big help in preparing the work, and the ready contribution of similar material from the European Association of Geoscientists and Engineers, the Geological Society, the American Association of Petroleum Geologists and the Canadian Society of Exploration Geophysicists is gratefully acknowledged.

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17 PART I Basic topics and 2D interpretation

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